A screenshot of Microsoft's Outlook email client for the Web. / Microsoft

by Brett Molina, USA TODAY

by Brett Molina, USA TODAY

The company announced it has completed the transition from the email service to the new Outlook.com, which now boasts more than 400 million accounts.

"We've been focused throughout the upgrade on ensuring that every customer has a great experience," says Dick Craddock, Outlook.com's group program mananger. "We've been overwhelmed with the response so far."

Along with a more modern interface, the Outlook email client introduces several features, such as two-factor authentication, an updated calendar and app and integration with cloud service Skydrive and Skype.

Users with Hotmail addresses can continue to use those accounts as long as they choose. However, Hotmail address owners can opt to claim an Outlook email address.

"Our belief is that as people start using the new experience, they will come to love it even more than they loved Hotmail," says Craddock. "We are keenly listening to what our customers have to say, and we'll make the right set of adjustments to ensure that we make the experience as great as it can be."